Morphological, histochemical and morphometric study of the myotomal muscletissue of the pacu (Piaractus mesopotamicus Holmberg 1887: Serrasalminae, Characidae, Teleostei)

Histochemical, ultrastructural and morphometric methods were used to study growth patterns of red, pink and white muscle fibres and their relation to body weight and total length in the fast-growing freshwater fish Piaractus mesopotamicus Holmberg. The correlations amongst body weight, body length a nd diameter of red, pink and white fibres were low. From 10-15 to 40-50 cm, body weight increased 102.7 times, while the diameter of each type of fibr o increased by factors of 0.94, 0.74 and 0.70, respectively. Muscle fibres revealed different morphological and histochemical stages of maturation. Th e frequencies of < 20 <mu>m fibres of red, pink and white muscle tissue in the youngest and oldest classes were 64.5 and 11.0, 38.2 and 7.7 and 24.0 a nd 1.4%, respectively. In 30-40 cm fish, the frequency of < 20 <mu>m fibres in the red and pink tissue was 24.5 and 25.5%, while in the white tissue i t was 11.5%. During sexual maturity (40-50 cm), the recruitment of < 20 pm fibres in white muscle was 1.4%. Muscle fibres of this species showed conti nuous growth by both hyperplastic and hypertrophic mechanisms, and hyperpla sia was particularly active in the juvenile phase.